1. The Field of the Invention
The present invention relates to valve apparatus and methods for delivering a predetermined amount of a pressurized fluid. In particular, the present invention relates to positive-fill metering valves and methods for repeatably delivering a precise amount of an aerosol.
2. The Prior Art
Various different types of metering valves have been developed in the art for dispensing aerosols from aerosol containers. Such metering valves have found particular utility in the administration of medical formulations which can be mixed with a liquified gas propellant and delivered to a patient in an aerosol.
As will be appreciated, when aerosol container devices are used to dispense medications, it is often quite important that a precise, predetermined amount of the medication is dispensed with each successive dose, so that each dose delivered will contain the proper amount of medication needed to effect the desired physiological response. For example, if the aerosol medication dispensed is intended to prevent an asthma attack in an asthma patient, an incomplete dosage delivered to the patient could well mean that the attack would not be arrested. As will be appreciated by those of ordinary skill in the art, there are many other instances where the delivery of a precise and a reliable amount of medication is very important and very desirable.
Thus, in order to provide some control over the amount of aersol formulation released in each dosage from the aerosol container, a metering valve was developed having a metering tank or chamber to define the amount of formulation dispensed in each dosage. Obviously, the precise dosage delivered from such a prior art metering valve is dependent upon the dimensions and other physical characteristics of the metering tank as well as the physical conditions to which the formulaton is subjected within the metering tank.
In the course of normal operation, the metering tank of a typical prior art metering valve is full of the aerosol formulation before the valve stem is depressed to release a dose. Thus, the metering tank must be refilled with formulation after discharging one dose so as to ready the metering valve for the discharge of the next dose. As a result, except for the brief moment during dosage discharge, the metering tank is full of formulation at all times so as to be ready to dispense a dose as the user's needs require. Further, the passageways leading from the bulk of formulation in the aerosol container to the metering tank are often narrow and tortuous. Unfortunately, many disadvantages have been experienced in the usage of prior art metering valves having these features.
For example, because the prior art metering tank is filled with aerosol formulatoin at all times in between dosages, the formulation in the metering tank is often adversely affected. First, a loss of prime may result. Because of temperature changes, vibrations, or the particular orientation of the metering valve during storage, the formulation may drain out of the metering tank in between dosages, thereby resulting in a loss of its prime. Even upon simple standing for extended periods of time between dosages, the metering tank may become empty, thereby requiring activation of the valve stem several times before the valve will deliver a full dose of formulation. As will be appreciated, such a purging procedure is wasteful of the aerosol formulation, and there is no way of determining how many doses must be dispensed before the valve is fully primed and prepared once again to deliver a full dose of formulation. Moreover, when loss of prime occurs, a vapor lock may form in the metering tank which prevents further formulation from entering into the metering tank.
Another related disadvantage which is often encountered in such prior art metering valves is the erratic delivery of medication where suspension formulations are involved. Many medications are comprised of a suspension of various chemical components which will either sink or float in the liquified gas propellant used to form the aerosol. Thus, depending upon the relative density of the various components in the aerosol medication, certain components in the medication may either float or sink into or out of the metering tank, depending upon the storage position of the metering valve. As a result, the dosage subsequently delivered to the patient will not contain the proper composition and concentration of components intended to be delivered. Further, if the aerosol container is shaken, gas bubbles may form. If these bubbles find their way into the metering tank, the next dose delivered will contain a mixture of formulation and bubbles instead of a full dose of formulation.
Still another problem often encountered with prior art metering valves is the undesirable phenomenon known as "tail-off." Tail-off refers to the fact that, when the aerosol container gets close to being empty, dosage delivery becomes erratic. Thus, the last few dosages delivered often contain less than the full amount of formulation.
Yet another problem encountered in the usage of prior art metering valves is the undesirable phenomenon known as "holdup." Holdup refers to the excess formulation which is left in the aerosol container after the last possible dose has been discharged from the metering valve. Naturally, when the formulation being delivered is significantly expensive or valuable, the holdup or amount of formulation left in the aerosol container and subsequently discarded becomes important. Unfortunately, the holdup experienced in many prior art aerosol dispensing devices often represents many doses which are ultimately wasted.
In an attempt to address some of the problems of the prior art, a retaining cup or drainage cup has been incorporated into some metering valves. However, such retaining cups have not satisfactorily solved the problems of loss of prime, erratic delivery of medication comprising suspension formulations, tail-off, and holdup. For example, temperature changes and vibrations within the metering valve may still cause the formulation to be expelled from the retaining cup, thereby resulting in erratic medication delivery. Further, retaining cups still experience significant tail-off and holdup.
From the foregoing, it will be appreciated that it would be a significant advancement in the art to provide metering valves which do not experience significant loss of prime or deliver erratic compositions and concentrations of medication. Further, it would be a significant advancement in the art to provide metering valves wherein the problems of tail-off and holdup are substantially eliminated. Moreover, it would be an advancement in the art to provide metering valves which deliver a precise, predetermined amount of a pressurized fluid or aerosol such that each successive dose is of virtually the same amount and composition, thereby providing for repeatable dosages even after standing or storage for a significant period of time. Such metering valves and methods for dispensing pressurized fluids such as aerosols are disclosed and claimed herein.